Recording media of the generic type stated at the outset are known in particular for audio tapes having HiFi capabilities and a low noise level. Use is made of the fact that in particular low-frequency signals magnetize the total magnetic layer whereas high-frequency signals are recorded substantially only at the tape surface.
The recording properties of a magnetic recording medium are essentially determined by the coercive force, the residual induction and the particle fineness of the magnetic pigments used. Thus, the high-frequency signal level can be improved within certain limits by increasing the coercive force, the low-frequency signal level by increasing the residual induction and the noise behavior by increasing particle fineness of the magnetic pigment. Since these pigment properties cannot however be equally pronounced in an individual pigment type, attempts have been made to achieve the desired properties by combining suitable pigments within an individual magnetic layer or by combining several magnetic layers having different pigment types.
DE-A 19 38 006 describes a multilayer tape comprising chromium dioxide as the main component of the upper layer. The object of the invention is to reduce the magnetostrictive behavior of volume-doped cobalt ferrite pigments in the lower layer. According to this publication, audio tapes, for example for the IEC II bias setting, can be produced thereby.
In DE 25 56 188, improved electroacoustic properties are achieved by combining two magnetic layers, the lower layer adjacent to the nonmagnetic substrate containing a chromium dioxide having a BET surface area of from 15 to 25 m.sup.2 /g and the upper magnetic layer containing a chromium dioxide having a BET surface area of 26-36 m.sup.2 /g. An audio tape having improved sensitivity at long wavelengths is thus obtained.
DE 31 48 769 claims a double-layer tape which has a longitudinal orientation of the magnetic acicular pigments in the lower layer and a perpendicular orientation of said pigments in the upper layer.
DE 26 47 941 describes a multilayer tape having an upper layer comprising an iron oxide having a coercive force of 29-32 kA/m and a residual induction of less than 0.15 mTesla and a lower layer comprising a magnetic pigment having a coercive force of 20-40 kA/m and a residual induction of more than 15 mTesla. The aim here is to obtain a standard audio tape having improved noise characteristics.
DE 39 14 565 discloses a double-layer tape in which CrO.sub.2 having a coercive force of at least 48.6 kA/m and a BET value of at least 28 m.sup.2 /g is present as magnetic pigment in the upper layer and an acicular cobalt ferrite having a coercive force of 44-56 kA/m and a BET value of 26-34 m.sup.2 /g is present in the lower layer.
Furthermore, DE 23 05 247, 24 34 940, 28 26 565, 30 04 771, 32 19 779 and 35 14 649 and the European Applications 0 090 052 and 0 090 053 disclose magnetic recording media which have a double-layer structure which is intended to improve the playback properties.
Investigations by the Applicant have shown that the output level in the medium frequency range in many high-quality audio tapes of the IEC-II type is reduced in comparison with the low frequencies, resulting in audible linear distortions, especially in conjunction with the Dolby noise suppression system.